I. Field of the Invention
The present invention relates to a temperature control apparatus for maintaining an object such as a sensor or the like in an automobile engine at a predetermined constant temperature.
II. Description of the Prior Art
Conventionally, it is known in practice that a heater may be disposed to heat a sensor at a predetermined temperature so as to decrease an error in a detected output from the sensor when the detected output is temperature dependent. Among methods for heating the sensor, a method is proposed wherein a constant voltage is applied to the heating portion such as a heater. However, according to this method, since it takes a long period of time to heat the sensor to the predetermined temperature, the sensor cannot be immediately started. Furthermore, if a heat dissipation condition of the sensor is not good, the sensor becomes overheated. Conversely, if the sensor is exposed to an atmosphere where air blows strongly around the sensor, the sensor becomes overcooled, resulting in inconvenience. In order to solve these conventional drawbacks, another conventional method for heating the sensor is proposed as a constant temperature heating control method wherein a sensor temperature is detected and power is supplied to a heater so as to keep the sensor temperature constant in accordance with the detected sensor temperature. According to this method, the sensor temperature can be kept constant. However, a temperature rise rate may become too high, resulting in inconvenience. In particular, in order to perform constant temperature heating control by disposing a microheater on one major surface of a substrate and a gas sensor on the other major surface thereof, a method for controlling the sensor temperature is proposed wherein the heater comprises a material having a high resistance temperature coefficient, and a heater temperature is detected by an electric resistance of the heater without using a separate temperature sensor. This method has advantages in that temperature detection can be immediately performed, that the input power can be increased, and that the temperature rise rate can be high. However, according to the sensor described above, if the temperature rise rate is too high, the substrate may crack due to thermal strain. Furthermore, the heater and the gas sensor are degraded, thus presenting various problems.